HPPase

HPPase, short for H+-translocating pyrophosphatase, is a membrane-bound enzyme that catalyzes the hydrolysis of inorganic pyrophosphate (PPi) to two inorganic phosphate molecules (Pi). In the reaction PPi + H2O → 2 Pi, the released energy is coupled to the active transport of protons across a membrane, generating or contributing to a proton motive force used for various cellular processes.

Distribution and localization of HPPases vary across life forms. They are found in bacteria, archaea, and plants, and in these organisms they localize to membranes such as the inner bacterial membrane, plant plasma membranes, or vacuolar membranes. In plants, H+-translocating pyrophosphatases can participate in vacuolar acidification and other processes linked to storage, osmoregulation, and stress responses. In bacteria and archaea, these enzymes contribute to energy management and proton motive force, especially under conditions where PPi is abundant or ATP is limited.

Structure and cofactors: HPPases are large multi-pass integral membrane proteins, typically containing six to twelve transmembrane helices. They possess conserved catalytic residues in the soluble regions that coordinate metal ions, with Mg2+ or Mn2+ commonly required for activity. Some isoforms show sensitivity to monovalent cations such as K+, which can influence catalytic efficiency.

Mechanism and significance: the hydrolysis of PPi by HPPase is directly linked to proton translocation across the membrane, thereby helping establish or maintain the proton motive force used for transport work and, in plants, for vacuolar functions. The precise catalytic mechanism is actively studied, but the enzyme is recognized as a key energy-conserving catalyst in PPi-rich conditions.